2o6 BOTANICAL GAZETTE [march 



first 6 ft. a ratio of i . 5 or lower was found, and commonly in one 

 or more of the foot sections a ratio as low as i . i was encountered. 

 After droughts of unusual severity the whole of the subsoil to a 

 depth of 6 ft., and in some cases of 12 ft., showed a ratio of 

 approximately I. o. 



3. There was no appreciable further reduction of the moisture 

 content when, after the subsoil had been reduced to this very dry 

 condition, there followed a 4 or 5-month period of practically rain- 

 less autumn and winter weather. After such droughts the surface 

 foot was found but little drier than the subsoil. 



4. The subsoils of the humid prairies, on the contrary, showed 

 no distinct reduction of the moisture content through a greater 

 depth than 5 ft., and even in this a ratio as low as 1.2 or 1.3 

 appeared only under the severest drought conditions. The normal 

 moisture condition in the deeper subsoil (6-20 ft.) appears to 

 correspond to a ratio lying between 2 . o and 2 . 4. 



5. The dry condition of the deeper subsoil so common in the 

 semi-arid prairies is to be attributed to the presence of perennials 

 with a vertical root range of 15 ft. or more, while the moist con- 

 dition characteristic of that of the humid prairies is regarded 

 as evidence that the roots of the native vegetation are but 

 little developed below the fifth foot. The occurrence of areas 

 in the semi-arid prairies, even after a severe drought, in which 

 the subsoil below the sixth foot is quite moist, is to be attributed 

 to the absence or fewness of deep rooted perennials in such 

 places. 



6. After the subsoil at any level has been exhausted of the water 

 in excess of the hygroscopic coefficient it remains in this dry condi- 

 tion until the precipitation conditions are sufficiently favorable to 

 raise the ratio to 2.0 or upward throughout the whole distance 

 from the surface down to the level in question. Accordingly 

 during many wet periods following droughts the upper moistened 

 portion of the subsoil will be isolated from any deeper lying moist 

 layer by a zone in which the subsoil is too dry to permit of the 

 penetration of plant roots. 



7. While in the semi-arid prairies after protracted droughts the 

 moisture conditions in the first 6 ft. are quite uniform, under more 



